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Angel Angel
Asked: 2020-12-10 02:22:04 +0800 CST 2020-12-10 02:22:04 +0800 CST

How can I call C++ code from C? (member functions)

  • 772

This is what I've used to call non-member functions in C++ from C code and it works fine.

file.cpp

extern  "C" void f(int);

    void f(int i){

        printf("c++!\n");
    }

main.c

#include <stdio.h>
#include <stdlib.h>

void f(int);

int main()
{
    printf("Hello world!\n");

    f(1);
    return 0;
}

shell:

Hello world!
c++!

How can I call member functions from C?

c++

1 Answers

  1. Best Answer

    When you make code compatible between C and C++, that code must have features common to both languages. This implies, among other things, that the code must not have classes, as well as other elements specific to C++.

    If you need C to end up calling C++'s own elements you need to create an API (in C++) that encapsulates all object logic, which is not recognized by C, and allows its manipulation through the use of simple functions.

    An example to manage a string. I don't have a C compiler at hand, but except for some small adjustment I understand that it should work

    // header file
#ifdef _cplusplus
#define EXTERN_C extern "C"
#else
#define EXTERN_C
#endif

typedef enum object_type
{
  ot_undefined,
  ot_string
} object_type;

typedef struct objData
{
  void* ptr;
  object_type type;
} objData;

EXTERN_C objData new_string();

EXTERN_C int set_string(objData obj, char* str);

EXTERN_C int get_length(objData obj);

EXTERN_C void delete_obj(objData* obj);

#undef EXTERN_C

// source file

objData new_string()
{
  objData toReturn;
  toReturn.ptr = new std::string;
  toReturn.type = ot_string;
  return toReturn;
}

int set_string(objData obj, char* str)
{
  int to_return = 0;

  if( obj.type == ot_string )
  {
    static_cast<std::string*>(obj.ptr)->assign(str);
    toReturn = 1;
  }

  return = toReturn;
}

int get_length(objData obj)
{
  int toReturn = -1;

  if( obj.type == ot_string )
  {
    std::string str* = static_cast<std::string*>(obj.ptr);
    toReturn = static_cast<int>(str->length());
  }

  return toReturn;
}

void delete_obj(objData* obj)
{
  bool ok = false;

  switch( obj.type )
  {
    case ot_string:
    {
      ok = true;
      std::string* str = static_cast<std::string*>(obj->ptr);
      delete str; // Si hago delete de void* no se invocan los destructores
      break;
    }
  }

  if( ok )
  {
    obj->ptr = 0;
    obj->type = ot_undefined
  }
}
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